As a mixer for a musical you are a bit of an alien to the theater. All of the other jobs -- from dressers to follow-spot operators -- are well established in the history of theater but amplification and live sound mixing are still new to the trade. We are more from the world of live music, from concerts and clubs, then we are from the world of greasepaint and limelight.
And you are physically isolated. Which you share with the lighting tech, and often the Stage Manager -- but they have headsets linking them electronically to the rest of the production. In the long spaces between cues there is chatter on headset -- news and gossip from backstage, and the social grease of people working long hours together.
They also have a nice little booth to hide in; you are usually alone on the floor in full view of the audience.
Of course it goes without saying that the Stage Manager has the ultimate loneliness; the loneliness of Command (insert your favorite Captain Kirk scene here). Our responsibility is not as heavy, but it is no small weight in itself.
We are the final link between actors/musicians and the ears of the audience. Some times this makes you the mastering engineer; the person responsible with taking all that effort and heart that so many people put into the music and giving that final polish to make it the best it can be. Other times you are like the last driver with a clear chance to avoid the accident.
And you switch between these modes with blinding speed. At one moment, you will be gently riding a mic to put that last little bit into the crescendo of an emotional number. And then there is a screech of sound and in an instant you are in damage control mode, force to make a choice between multiple unpalatable alternatives...without any time for deliberation.
On a very good night, someone might give you an atta-boy for responding quickly to plug that popped out of a DI in the pit and subjected the audience to the growling buzz of unfiltered 60-cycle. On a very, very good night, you might get a compliment along the lines of, "We didn't hear any noise or popping this time." No matter how many problems you solve before the curtain opens, no matter how many prophylactic measures you take (like subjecting a poor actor to multiple mic changes just because you thought you heard something in their mic), no matter how quick and how effective you fixed, charted around, or otherwise ameliorated a problem, the only feedback you will ever get is on the ones that slip through.